Population dynamics is a funny thing. Good numbers of adults is a good thing, but it can lead to overpopulation by larvae (relative to what the local ecosystem will support), and larger numbers of larvae can be a bonus for predatory animals and pathogenic organisms. That can all lead to a fall in numbers of adults next year. It is all about stability in the ecosystem, and how fine a balance that can be. A little variation might be OK, but too much or too little of anything can lead to disproportional ramifications. Of course, one can take a slightly longer-term view and say that the swings are a manifestation of a dynamic stability, which may be true. But however you look at it (and it is really difficult to be cogent of all the sides of the equation), ecosystems are complex. Best thing, unless one happens to be a professional ecologist, is to take the simple view - to enjoy the beauty and bounty nature offers, when she does. Love 'em burnets.